When processing the write command, the console scans for the magic strings "Command:" and "Ready" which occur during the boot process. When this occurs, Multics (BCE) is waiting for the operator to press the ATTN key, but this magic string is the only signal that BCE issues, so the AUTOINPUT code has no way to know that input is expected. When the console sees this string being printed, it activates 'attn_unit' with an appox. 1 sec delay. 'attn_unit" sets the console interrupt flag to trigger a console read command.
There is also a sim_activate for the mount unit, but that code is no longer used.
The boot process uses a sim_activate to set an interrupt a very short period of time after the IOM BOOT button is pressed.
sim_activate is used to start the IOM on the receipt of a CIOC command.
sim_activate is used several places.